Among engines, there is an engine capable of using fuel containing alcohol such as ethanol. When fuel containing alcohol is used, it is possible to increase the engine efficiency by increasing the effective compression ratio because the octane number of fuel containing alcohol is high. On the other hand, as the alcohol concentration is increased, vaporization performance of fuel is deteriorated. This may deteriorate the cold start performance. In view of the above, Patent Literature 1 discloses directly injecting fuel into a cylinder in a latter stage of a compression stroke in which the in-cylinder temperature is increased when an engine is started in a cold state.
When fuel is changed from fuel having a high alcohol concentration to fuel having a poor property (e.g. fuel having a low alcohol concentration, or fuel containing only gasoline having a poor property) while keeping the effective compression ratio unchanged, pre-ignition is likely to occur. In particular, when an engine is started after an ignition switch is turned on from a state that the engine is stopped, a lead time for self-ignition increases in a predetermined low speed range (e.g. 200 rpm) when fuel injection is started. This may likely to cause pre-ignition.
Particularly, some of the vehicles may be used in a district where only the fuel having a poor property is available. In view of the above, how to prevent pre-ignition is a task to be solved. Note that in the aspect of preventing pre-ignition, for instance, it is proposed to operate a valve timing variable mechanism (means for changing an opening/closing timing of an intake valve) when occurrence of pre-ignition is predicted for lowering the effective compression ratio. However, a valve timing variable mechanism of a hydraulically driven type cannot secure an operation thereof unless an engine is started. Therefore, if a valve timing variable mechanism is of a hydraulically driven type, it may be impossible to lower the effective compression ratio at a start time of the engine when pre-ignition is likely to occur.